Bone marrow

Normal bone marrow is divided into red and yellow marrow, a distinction made on the grounds of how much fat it contains.

Red marrow is composed of:

haematopoietic cells

supporting stroma

reticulum (phagocytes and undifferentiated progenitor cells)

scattered fat cells

a rich vascular supply

Conversely, yellow marrow has all the same constituents as red, except that fat cells make up the vast majority, with resulting poor vascularity. Distribution varies with age and from one individual to another, but should be symmetric.

During infancy red marrow occupies the entire ossified skeleton except for epiphyses and apophyses. Gradually red marrow 'retreats' centrally, such that by adulthood it is essentially confined to the axial skeleton (pelvis, spine, shoulder girdle, skull). Frequently the proximal humeri and neck of femurs have residual red marrow2.

In addition, islands of red marrow may be seen anywhere in the skeleton, typically in a subcortical distribution, often with central yellow marrow giving it a bull's eye appearance on axial imaging. Additionally red marrow is found in subchondral crescents again of the proximal humerus and femur.

Most of the above conditions (covered individually in the encyclopaedic section) affect the marrow diffusely. The exception is multiple myeloma which has a predilection for focal deposits, and Waldenstrom macroglobulinemia which causes infarcts.

The abnormal signal is due to replacement of the small amounts of fat cells normally found in red marrow, such that T1 signal will decrease to or below the signal from disc or muscle. T2 signal is more variable, but will in general increase when compared to muscle.

The leukaemias typically affect the metaphyses > diaphyses > epiphysese. Changes in the latter indicate a large tumour load, and therefore has prognostic implications.

Yellow-to-red reconversion, obviously, generate red marrow in abnormal distribution. Signal is therefore a very important aspect of correct image interpretation. It occurs in the reverse order to that of red to yellow conversion, and is seen in:

Red marrow reconversion can be difficult to differentiate from metastases in the spine. A useful pair of sequences is T1 in- and out-of-phase. If there is focal low signal on T1 in-phase this may be due to either pathology. However the scattered fat cells in red marrow cause marked signal loss on out-of-phase images. There is no such signal loss in metastases.